A plasma generating device such as a duoplasmatron creates an intense plasma between a cathode and an anode through an intermediate electrode. The plasma is intensified by the constricting action of an orifice in the intermediate electrode and the focusing action of a magnetic field between the intermediate electrode and the anode. Ions are extracted from this plasma at the anode aperture, as a result of an accelerating electric field created by raising the potential of the entire source relative to a grounded extraction electrode near the anode aperture. Cooling of the cathode, intermediate electrode, and anode is required to prevent excessive outgassing and oxidation. Most commonly, this is accomplished by circulating a liquid coolant through passageways in the source structure. This is undesirable because of the attendant design complications, the requirement of a heat exchanger, and the inconvenience of servicing the source. Cooling of these parts has also been done by forcing compressed air through similar passageways, but similar design complications are involved, and a source of compressed air is required.
In order to extract ions from the duoplasmatron source, a potential is applied to the entire source relative to some grounded extraction electrode. In prior designs, the source mating flange and the magnetic coil were also floated at this potential. Consequently, an intermediate insulator section was required to interface the source to any focusing optics, and the circuit that powered the magnet coil was required to float at the high potential.
The plasma forming gas source and valve, which are generally at ground potential, need to be electrically isolated from the gas inlet on the duoplasmatron which is at some high potential. This is accomplished by incorporating a ceramic tube between the gas inlet and the valve. In order to prevent a discharge inside the tube, the tube in past designs has been made long with a small inner diameter. This proves to be a relatively cumbersome design and results in excessive pressure drop through the tube.
An object of the invention is to provide a novel duoplasmatron-type ion source that is ultra-high vacuum compatible, and hence very clean; has a variable magnetic field produced by an integral coil that is at ground potential; has an ion current output that is very stable over time, and is mounted to a standard type flange at ground potential without the necessity of subsequent high voltage isolation.
Another object is to provide a novel cooling system for a plasma generating device.
Yet another object is to provide a novel gas conduit resistant to electrical discharge under very high voltage conditions.